This invention relates to a synthetic ester lubricating oil composition having suitable high temperature properties and hydrolytic stability and to a method for providing hydrolytically stable turbine oils. More particularly, this invention is directed to a synthetic ester lubricating oil composition containing an additive package comprising the combination of a selected tertiarybutylphenyl phosphate and a selected alkyl amine.
Lubricating oils containing synthetic esters as oil base stocks are well-known in the art. In fact, due to the unique physical characteristics of these materials, the synthetic ester lubricating oils have been widely used in those areas where the oils are subjected to extreme temperature variations such as in aircraft engines and the like. These ester base oils do not, however, inherently exhibit high load carrying ability and cannot be used, without modification, where a high degree of lubricity is required. Also, the synthetic ester oil compositions are subject to oxidative degradation and cannot be used, without further modification, for long periods of time under oxidizing conditions. It is known that this degradation is primarily due to oxidation, heat and hydrolysis of the ester base oil.
Hydrocarbyl phosphate esters are well-known metal passivators, load carrying and extreme pressure additives for lubricant compositions. Among the different phosphate esters employed in this manner are a number of triaryl, trialkaryl and trialkyl phosphates with tricresyl phosphate being particularly preferred and used in many formulations.
Disclosures of phosphate esters of this type in lubricant compositions can be found in U.S. Pat. Nos. 3,468,802; 3,780,145; 3,914,023; 4,064,059; 4,087,386; 4,141,845 and 4,179,386. Many of these patents further disclose the use of amines and particularly aryl amines as antioxidants for different lubricant compositions. Amine type additives have also been disclosed as storage and hydrolytic stabilizers in lubricants as noted, for example, in WADC Technical Report 59-379, January, 1959 entitled "Improvement of the Storage Life of MIL-L-7808 Oils" where the use of aliphatic amines such as 2,6-di-tert-butyl-.alpha.-dimethylamino-p-cresol is shown. Also, U.S. Pat. No. 3,914,179 discloses the use of aliphatic and aliphatic/aromatic amines of a selected type with 2,6-ditertiarybutyl-4-dimethylaminomethyl phenol (same as cresol compound above) being preferred.
While many phosphate esters, as described above, and particularly tricresyl phosphate have been used as additives for lubricants such as synthetic lubricating oils, problems of hydrolytic degradation have sometimes developed when using certain phosphates. Such a problem could exist when using tertiary-butylphenyl substituted phosphates in selected synthetic ester lubricating oils. Other additives including amines have been used in lubricant compositions, as noted above; however, many of such additives are not helpful in inhibiting hydrolysis while even some of such additives which do provide hydrolytic stability for certain compositions are not particularly suitable for high temperature applications because of properties such as high volatility. Accordingly, the need still exists for providing a synthetic ester lubricant composition which is hydrolytically stable and has desirable properties such as load carrying ability and lubricity and additionally is particularly useful in high temperature applications such as involved in the operation of aircraft engines.